Capt Johnson appealed to demonstrators to protest in the daytime to prevent a "tiny minority of law breakers" from deliberately sparking violence.

"Anyone who has been at these protests understands that there is a dangerous dynamic in the night: it allows a small number of violent agitators to hide in the crowd and then attempt to create chaos," he told journalists.

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Media captionLocal resident: "When they started throwing tear gas, I started running"

Those arrested include people from other states, including New York and California, Capt Johnson said. Correspondents say this suggests the protests are becoming more organised.

In an earlier statement, US Attorney General Eric Holder said he would "personally travel" to Ferguson on Wednesday to meet FBI investigators and prosecutors.

"I realise there is tremendous interest in the facts of the incident that led to Michael Brown's death, but I ask for the public patience as we conduct this investigation," Mr Holder said.

He added that the investigation into the shooting of 18-year-old Mr Brown was "a critical step in restoring trust between law enforcement and the community, not just in Ferguson, but beyond".

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Media caption"A small number of violent agitators... hide in the crowd and then attempt to create chaos" - Police Captain Ron Johnson